Over the past decade, we have made major strides towards determining the pathogenesis and testing a treatment for Leber's Hereditary Optic Neuropathy (LHON). We successfully expressed the wild-type human NADH ubiquinone oxidoreductase subunit 4 (ND4) of complex I in the nuclear genetic code. The protein was imported into the mitochondria by agency of a mitochondrial targeting sequence. The gene was packaged into next generation tyrosine to phenylalanine modified self-complementary adenoassociated virus (AAV) then injected into rodent eyes. FLAG-tagged wild-type human ND4 was detected quickly in most inner retinal neurons by 1 day post injection and it integrated into Complex I. Furthermore, in rodent eyes also injected with a mutant Gl 1778A ND4 homologue responsible for most cases of LHON, wild-type ND4 restored defective ATP synthesis, suppressed visual loss, reduced apoptosis of retinal ganglion cells and prevented demise of axons in the optic nerve that persisted long-term. The self-complementary wild-type ND4 injected at the relevant titer into the ex vivo human eye expressed in most retinal ganglion cells, suggesting that it will do so in our LHON patients. Primates vitreally injected with untagge ND4 had no adverse reactions, suggesting that this vector should be a safe and effective platform for clinical testing in LHON. Our goal in this application is to test the safety of AAV-mediated delivery of the human ND4 gene in a Phase I clinical trial of patients with mutated G11778A mtDNA and then move to a Phase II study to prove efficacy in the later years of this program. Phase I will consist of an open-label, unilateral, single-dose intravitreal injection of AAV-ND4 per patient in the worse eye in a dose-escalation study investigating the safety of three vector doses (5x10e9 vg, 2.46x10e10 vg and 1xlOel1 vg) in a small number of patients with molecularly confirmed Gl 1778A-mutated mitochondrial DNA who have chronic bilateral, severe visual loss for more than 1 year (Aim 1) or acute bilateral several visual loss for less tha 1 year (Aim 2), and then, lastly, in the eye with better vision but that we know is predestined to lose significant vision within 6 months from the onset of visual loss in the first eye (Aim 3).